Method of changing the moisture content of wood



Aug. 1965 F. H. MILLIGAN ETAL 3,199,213

METHOD OF CHANGING THE MOISTURE CONTENT OF WOOD Filed July 12, 1961 4Sheets-Sheet l MIVENTOES FREDERXCK H. MILLIGAN h lvm b CHARLES W. HOYT.

F. H. MILLIGAN ETAL 3,199,213

METHOD OF CHANGING THE MOISTURE CONTENT OF WOOD Aug. 10 1965 Filed July12, 3.961 4Sheets-Sheet2 DRYING TIME FOR SECTIONS OF WOOD /3 INCH THICKALSO SHOWN ARE WOOD SURFACE TEMPERATURES) MOISTURE CONTENT VS.

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METHOD OF CHANGING THE MOISTURE CONTENT OF WOOD Filed July 12, 1961 4Sheets-Sheet 5 Ioo I MOISTURE CONTENT vs.

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*- 2 LL! Z 8 o m 10 268 F D: J 5 s 6 z 0 as? F o I 2 s 4 DRYING TIME-MINUTES lNl/EATOQS FREDERICK H. MILLIGAN CHARLES w. HOYT rray/5V; v

4 Sheets-Sheet 4 F. H. MILLIGAN ETAL 6 FREDERICK H. MILLIGAN CHARLES W.HQYT METHOD OF CHANGING THE MOISTURE CONTENT OF WOOD Aug. 10, 1965 FiledJuly 12, 1961 TIME HOURS B B DF w W L m nU M O m N m B W o B om V B FL 0O B Y M U 4 D BFE L o 6 IR E U B M D RT OR E D AA E NM 0 BY W I H w H OPmm M Y Y NM? flw D R "4. @E MO D m WmM T 5 m mmA cL E 8 ml. A V T P N om M w E IG R W R N A .L L 0 i N C A W E T A OW B N L B l O T @E 0E 0mm VN T TR R M A Y q A H i T HE P H 1 m m w m wm 11- E 0/ v H D M W M O NOaw A B M H H h p 0 M L S L L O a a a w mw m fl R R R T #0 W T B 0 4 Tom01.. 67 m a m. 9 1 3 8 O A 3 a. A A 3 M 4 Q L Q Y A u 7 3w E W -l'lu 6 54 3 2 O O 0 w w 3 m 9 P2528 E3202 9 IiTO United States Patent 3,199,213METHOD (9F CHANGING THE MOHSTURE CGNTENT 0F WQ'OD Frederick H. Miiligan,Port (Ioquitlam, British Columbia, Canada, and Charles W. Hoyt,Cambridge, Mass, assignors, by direct and mesne assignments, to QrownZellerhach Canada Limited, Vancouver, idritish Columhis, Canada, acorporation of the Province of British Columbia, Canada Filed July 12,1961, Ser. No. 123,535 8 Claims. ('Cl. 34-133) This invention relates toa method of changing the moisture content of wood, that is, to reduce orto increase the moisture content to a desired point.

The term wood as used herein is meant to include wood in the form ofstrips, sheets, veneers, boards, shingles and box material whetherproduced from logs, cants, blocks, or other pieces of wood by sawing,slicing, peeling or splitting, and it also includes products commonlyknown as fiakeboard, particle board, hardboard, plywood and compositionboards, the materials of which are primarily pieces or particles of woodor other lignocellulosic fibre bonded together naturally or by means ofresins or glues. All of the foregoing materials are, of course,generally planar in configuration having two major planar surfaces andlesser edge surfaces.

Standard methods of changing the moisture content of wood are generallyreferred to as air drying, kiln drying, oven drying, seasoning, curing,conditioning or humiditying. In most cases, the moisture content of thewood is reduced; although in the manufacture of such products asflakeboard, particle board, fiber board and some composite boards, it isoften necessary to increase the moisture content as one of the finalsteps in manufacture.

Seasoning, curing, conditioning and drying are all terms which refer tothe establishment of desired mechanical, physical and chemicalproperties in wood and wood products. Conditions of time, temperature,circulation rate and humidity when tempreatures are below 212 F. governthe extent of the moisture change and also the way the change is broughtabout, which in turn affects the properties of the wood or woodproducts.

Air drying is accomplished simply by stacking the material in a yard orunder a shed for a period of time long enough to permit evaporation ofmoisture to take place.

Kiln drying, or oven drying, as presently practiced, is essentially aprocess of heating the wood or wood product in a closed cabinet orchamber to accelerate the removal of internal moisture by a circulatingatmosphere. Regulation of temperature and humidity is used to controlthe degree and rate of moisture removal.

Conventional methods and equipment such as kilns, ovens and humidifyingchambers presently used for changing the moisture content of wood andwood products are faster and subject to better control than is airdrying. Drying lumber in a kiln, for example, is a matter of daysinstead of weeks or months. Drying thick softwood veneer takes tominutes in the roller type oven dryer widely used in the plywoodindustry in contrast to the several days that would be required for airor loft drying. Conditioning or increasing the moisture content offlakeboard, particle board and fibre boards of all densities isaccomplished in a few hours in a humidifying chamber as compared to daysor weeks in a loft or warehouse where conditions of temperature andhumidity approximate that of the prevailing weather.

Conventional wood drying equipment operates at temperatures ranging from100 F. to 425 F. For example, lumber and shingle dry kilns commonlyoperate at temperatures below the boiling point of water, while mostplywood veneer dryers operate at a range of 300 to ice 425 F. Thesedryers also circulate the atmosphere at low velocities of the order of600 f.p.m. in lumber and shingle kilns and 600 to 3000 f.p.n1. in veneerkilns with the flow of air parallel to the surface of the drying wood.

Drying wood and wood products at temperatures in excess of thosementioned above has been attempted in the past. However, it has beenasserted that use of high temperatures of the drying atmosphere as atechnique for reduction of drying time also results in reduction ofstrength, softening, chemical changes, and drying stress defects such ascase hardening, checks, splits, warping and honeycomoing of the wood.

In order to avoid harming or discoloring the wood, it is advisable notto allow the surface temperature to rise above 350 F., and for somespecies it is safer to keep this temperature below 300 F. Both time andtemperature are involved in the harming and discoloring of wood. Woodmay be subjected without ill effects to much higher temperatures for ashort period of time than for substantially lower temperatures andextended periods of time. However, there are some occasions on which aslight discoloration would not be objectionable. This would be the case,for example, when the veneers are used with a glue other than thephenolic resin used for producing exterior grade plywood.

The main object of the present invention is the provision of methods fordrying, humidifying or equalizing the moisture content of wood which arefaster, more economical and provide closer control of the conditionsaffecting the chemical, physical and mechanical properties of the woodbeing processed than is possible with existing methods.

Another object is the provision of a method of changmg the moisturecontent of wood by the utilization of high temperatures without harmingor discoloring the wood.

The method according to the present invention, contrary to generalopinion, utilizes high temperatures to dry wood without any harmfuleffects. It has been found that it is not the high temperature of thedryer atmosphere per se that directly causes the wood defects referredto above, but it is the high temperatures induced in the wood byexposure to the high temperature atmosphere or contact with parts of thedryer that are at high tcmperatureQ In the present method, the moisturein the wood is evaporated exceedingly rapidly, producing an evaporativecooling effect at the wood surface. This enables the wood to be dried toa low moisture content by a high temperature atmosphere withoutexceeding safe wood temperatures. In order to create high rates ofevaporation, it is necessary to circulate the drying atmosphere over thesurface of the wood at high velocities. This is preferably accomplishedby directing hot gas against the surface of the wood at substantiallyright angles thereto. The hot drying medium is directed through one ormore nozzles against the surface of the wood, but it is preferable todirect it against the wood by a plurality of nozzles throughout theentire surface of said wood. The previous research in high temperaturedrying has been done on simulated conventional equipment whichcirculates the drying atmosphere parallel to the wood surface at lowvelocities. Low velocity, parallel-flow does not evaporate the moisturerapidly enough to result in a significant lowering of the woodtemperature. When the high temperature atmosphere is directed through aplurality of nozzles, the moisture-laden air or gas can get away quicklyfrom the surface. In other words, the gas wipes the moisture off thesurface without unduly raising the surface temperature. As the moistureis wiped olf the surface, it is replaced by other moisture from theinterior of the wood. What is actually happening is each impingingstream of hot gas against the surface of the wood substantially norp beused to obtain satisfactory results.

7 3 mal thereto strips oif or scrubs the moisture-laden boundary layerof the wood to cause rapid evaporation of moisture from said surface andsimultaneously cool the surface. Thus, the hot gas stream is removingthe moisture and at the same time setting up a cooling action thatprevents the wood from being harmed in any way. One of the advantagesresulting from the use of the present method is the evenness of thetreatment over the entire surface of the wood. In the preferred form ofthe process, jets of hot gas are impinged against both the majorsurfaces of the wood throughout the entire areas of said surfaces. Thisresults in the wood being dried evenly throughout both of the majorsurfaces thereof. Use'of the prior parallel-flow types of dryer resultsin uneven treatment of the wood. For example, if the dryer is of'thecross-circulation type, the inlet side of the dryer will be at a highertemperature than the outside side thereof. This arrangement can resultin large variations in the moisture content of the dried product.Furthermore, with the prior dryer, it is difficult to control thevelocity of the drying atmosphere over the wood surface. Irregularitiesin the flow passages around the wood deflect the gas stream with theresult that the wood receives an uneven treatment. With the presentmethod, the drying atmosphere'is directed in a plurality of jets againstthe entire wood surfaces substantially at right angles thereto so thatevery part thereof receives exactly the same treatment.

' The basic method of changing the moisture content of wood inaccordance with this invention comprises impinging atleast one stream ofa moisture conditioning gaseous medium against the surface of the woodsubstantially normal thereto at a velocity of at least 1000 f.p.rn.

harming or discoloring the wood. For example, for dry- 4 ing purposes,temperatures of from 250 to 750 F. may It is believed that highervelocities and temperatures may be used, but they are at present notconsidered economic since special materials are required in theequipment for strength at high I With uniform moisture distribution, theaverage residual moisture content can be set only slightly below themaximum allowable level. On the other hand, with a wide variation inresidual moisture content, the average must be well below the maximumlimit, and the net effect is that more moisture must be removed fromwood dried in a conventional dryer than from wood dried by the presentprocess; Furthermore, the wide distribution results in overdrying withresulting degrade to a substantial part of the total quantity dried. I

It has also been found that the moisture content wood may be'raised veryquickly and accurately by impinging one or more jets, preferably aplurality of jets, of a gaseous medium against the wood surfacesubstantially normal thereto at a velocity of at least 1000 f-.p.m.,said gaseous medium having a dry bulb temperature less than V chambersso thatboth surfaces of each piece of wood are 212 F. and a wet bulbtemperature adjusted to establish the desired equilibrium moisturecontent. this method may be used to balance the moisture content of woodwhich initially is much wetter in some areas than in others.

In the accompanying drawings:

FIGURE 1 is a side elevation of one form of apparatus for carrying outthis method, most of the near wall of the apparatus being broken away,

FIGURE 2 is an enlarged fragmentary section taken on the line 2-2 ofFIGURE 1,

FIGURE 3 is an enlarged fragmentary section taken on the line 33 ofFIGURE 1,

FIGURE 4 is an enlarged vertical section taken on the line 4-4 of FIGURE1,

FIGURE 5 is a graph showing the moisture content reduction of woodsections /s" thick at specified drying medium temperatures-andvelocities, indicating the time required and the surface temperature ofthe wood,

FIGURE 6 is a graph showing the time required for reducing the moisturecontent of a wood section A" levels for a range of wet and-dry bulbtemperatures, and

FIGURE 8 is a graph showing the time to humidify A" hardboard, comparingdrying by the present method with drying by standard procedures.

FIGURES 1 to 4 illustrate apparatus for. carrying out this method. Adryer 10 has one or more drying sections, the illustrateddryer'including drying sections or zones 12, 13 and 14. The apparatusincludes an upper casing 13 spaced above a lower casing 19 to form athin passage 21 therebetween. 7 Upper and lower conveyors 23 and 24 haveadjacent horizontal runs 26 and 27 travelling in the direction of arrow28 inFIGURE 1 into, through and out of passage 21. Conveyors 23 and 24are designed to permit as much gas or ,air'as possible to passtherethrough, and are preferably formedof wire mesh having relativelylarge openings therein, as shown. These conveyors are moved by asuitable source of power, not shown, so that they move pieces of wood 30between the runs .26 and 27 thereof through the dryer 10 from itsentrance end 32 to the outlet end 33.

As drying sections 12, 13 and 14 are exactly the same, only one, namelysection 12, will now be described in detail. Upper casing 18v at dryingsection 12 forms an upper plenum chamber 36, the bottom of which isdefined by a ,wall 37 extending from an end 38 of the casing to aseparator 39 forming the inner end of said section. A plurality of jetnozzles 42 project downwardly from wall 37 and terminate just above thehorizontal run 26 of conveyor 23. Each of these nozzles opens intochamber 36;. Similarly, lower casing vl forms a lower plenum chamber 45between an end wall 46 and a separator 47. A wall 49 extends across theplenum chamber, and a plurality of jet nozzles 51 extend from said walland terminate near the run 27 of lower conveyor 24.

Hot gas, such as air, is directed at high velocities to upper andlower'plenum chambers 36 and 45 in any suitable manner, such as by pipesStand 55. -The hot gas is directed in a plurality of jets throughnozzles 42 and 51 to the upper and lower surfaces of wood pieces 30being moved through passage 21 by conveyors 23. and 2 It is preferablethat nozzles 42 and 51 direct the gas against fully covered by saidjets.

. The drying medium or gas escapes laterally from passage 21. Althoughthe gas may be allowed to escape to the surrounding atmosphere, it ispreferable to provide d-ucts 57 and 58 along opposite sides of theapparatus and Furthermore,

and 19 to close the sides of passage 21. The gas is taken away fromducts 5'7 and 53 through suitable pipes, and is either discharged toatmosphere or completely or partially recirculated in the system.

The operation of dryer is obvious from the abov description. The woodpieces 30 are moved by the conveyors through passage 2. In dryingsection 12, the hot drying medium or gas is directed in a plurality ofjets at high velocity to opposite faces of the wood pieces. Section 12only may be used, but under some circumstances, it is desirable tosubiect the wood to two or more diiferent sets of drying conditions and,therefore, drying section 13 and/ or 14 may also be used with section12. The operation is exactly the same in each drying section, and it isonly the drying conditions of the method that are changed. For example,the temperature and/or velocity of the drying medium in each section maybe different from that of each other section. The temperature andvelocity of the drying medium in the dryer are selected in accordancewith the condition and type of wood being dried.

TABLE 1 Time (minutes) to dry Douglas Fir heartwood Ai-inclz veneer from35% to 5% moisture content at various atmosphere temperatures andimpingement velocities Velocity, f.p.m. Temperature, F.

4. 38 3. 33 Y 2. 1. 70 1. 37 1.14 0. 99 0. 86 3. 97 2. 87 1. 85 1. 36 1.08 0. S9 0. 76 0. 67 3. 64 2. 53 1. 57 1. 1d 0. 89 0. 75 0. 63 0. 54 3.35 2. 24 1. 36 0. 98 0. 76 0. 62 0. 53 0. 46 3. 11 2. 04 1. 21 0. 85 0.66 0. 54 0. 46 0. 39 2. 88 1.85 l. 08 0. 76 0.59 0. 48 0. 0. 35 2. 73 1.70 0. 98 0. 68 0. 53 0. 43 0. 36 0.31 2. 54 1. 57 i 0. 89 O. 62 0. 48 0.38 0.33 0. 28

Table 2 shows some drying times for /8" Douglas Fir veneer in accordancewith this process and with standard practice. From this table it isapparent that this method can dry thin wood sections in as little as thetime required by conventional parallel circulation methods.

TABLE 2 Comparison of drying times for conventional and high velocitydryers Conventional Roll-Type 15- High Velocity Dryer Section VeneerDryer, Material Front operating at 320 F. Back operating at 450 F., 550F., 650 F., 750 F.,

340 F. 9,0001'.p.rn. 9,000 f.p.m. 0,000 f.p.m. 9,000 f.p.m.

%; Inch Douglas Fir Heart- 13. min 1.36 min 0.98 min 0.76 min 0.62 min.

wood, Average M. C. of 35%.

450 F., 7,000 f.p.m.

% Inch Douglas Fir Sap- 20 min 3.3 min.

wood, Average M. C. of 100%.

If different drying times are required in drying zones 12, 13 and 14,each zone may be provided with its own conveyors which are independentof the other conveyors so that the time the wood is in each zone may bevaried as desired. Another way is to move the wood in batches, in whichcase the conveyor speed would be adjusted f each drying zone. However,it is preferred to make the apparatus so that the temperature andvelocity of the gaseous medium may be controlled in each zoneindependently of the others, and to have the conveyors operate at aconstant speed. With this arrangement, the timethe wood is subjected toa given set of conditions is determined by the number of zones in whichsaid conditions exist. For example, one zone may have one set ofconditions, and the next two Zones another set of conditions so that thewood sections would be subjected to the second set of conditions longerthan to the first set.

5 is a graph showing a group of representative drying curves determinedby drying thin wood sections in accordance with this moistureconditioning method. For example, curve A shows that wood with aninitial moisture content of 35% can be dried to 5% moisture content in3.3 minutes in a dryer atmosphere of 350 F. impingin g against the woodat 3000 r".-p.m. Curve K shows that wood can be dried from 35% to 5%moisture content by the utilizing of a drying medium at 650 F. and 7000fpm. in 0.89 minute without raising the wood temperature above 305 F.The graph indicates wood surface temperatures with the dryer operatingat 650 F. and with it operating at 350 F.

The following Table i has been prepared from accumulated data and showsthe time required to dry thin Wood sections from 35% to 5 moisturecontent at various dryer atmosphere temperature and velocities.

The curves shown in FIGURE 5 provide the basis for drying schedulesapplicable in a commercial dryer. example, a single pass, ft. long dryer10 (with one drying section only) operating at 650 F. and 7000 f.p.m.(curve K) would dry thin wood sections from 35% to 5% MC. in 0.89 minuteat a conveyor speed of 84 f.p.m. A conventional roller-type, 5-line, 75ft. long counter-current circulation dryer operating in the range of 320F. to 340 F. would require 13.0 minutes (see Table 2) to dry the woodand the rate of output would be 5.8 f.p.m. per line or 29 f.p.m. for theentire dryer.

The curves presented in FIGURE 5 also provide the necessary informationfor establishment of zone conditions to secure minimum drying times whena specific limit is placed on allowable wood temperature and finalmoisture content. For example, it may be desired to dry thin sections ofheartwood from 35% to 5% MC. in min mum time without exceeding a woodtemperature of 305 F. Using the data in FIGURE 5, a dryer with twoindependent drying sections may be used as follows:

Zone 1650 F. at 1l,000 f.p.m. for 0.50 minute (curve M) Zone 2650 (curveK) This two-section schedule enables the wood to be dried in 0.71 minuteas compared with 0.89 minute for a single zone dryer operating at 650 F.and 7,000 f.p.m. Wood temperatures would not exceed 290 F.

The drying processes widely employed in the plywood industry use higheratmosphere temperatures at the dry end than at the green or wet end.Consequently, the driest wood is acted upon by the hottest gases. In thepresent process, the practice is reversed. That is, the high temperatureatmosphere is applied to the green or wet wood until the woodtempenature approaches the maximum allowable limit; then the wood entersa lower temperature F. at 7,000 f.p.m. for 0.21 minute For atmosphere.In prior dryers, evaporative cooling does not take place appreciably;however, in this process, the effectiveness and controlof the dryingoperation depends upon this phenomenon.

This process is also eifective in drying thin Wood sections containing ahigh percentage of moisture. FIGURE 7 TABLE 3 5%. Other combinations ofdry and wet bulb tempera- 'tures to establish a range of desiredequilibrum moisture contents are shown in FIGURE 7.'

Equilibrium moisture content can be reached by exposing the wood to airwhere there is natural circulation or by exposing to air with forcedcirculation at low velocities as in conventional kilns or conditioningchambers. However, reaching equilibrium moisture content in a lowcirculation rate atmosphere requires considerable tirne. In contrast,direct impingement of an atmosphere moving at high velocity and atproper conditions of temperature and relative humidity permitsestablishment of equilibrium conditions in the wood in much shortertimes than is possible by other methods now in use. For example, assumethe piece to be dried con- Boil/shcar stress, percent of wood failure ofplywood (3-ply) samples constructed with Douglas Fir veneers dried in ahigh velocity dryer Percent Moisture Dryer Tests Content Veneer DryingThickness, Time,

Inches Minutes Temp, Velocity, Boil IShear, Wood Initial Final Zone F.t.p.m. lbs/sq. in. Failurc, Percent Adhesive used in test panels wasAmerican Marietta AM5581 with Furafil 1005 as compounded and used forregular plywood production. Glue spread was 54 lb./M sq. ft. double glueline.

All test panels of 3-ply construction pressed at 200 p.s.i., 290F.; 3%min. for i o veneers; 4 min. for veneers.

Sometimes it is necessary to increase the moisture content of wood, andparticularly with flakebo'ard, particle board and fibre boards of alldensities. This procms may be utilized to increase the moisture contentof the wood. Increasing moisture content is known as conditioning orhumidification and can be accomplished by this process in much less timeand more uniformly than in conventional equipment. By maintaining thegaseous medium at temperatures of 200 F. dry bulb and 170 F. wet bulb(relative humidity about 50%), the moisture content of oven dry wood canbe raised to 5% moisture content in much less time than is requiredwithconventional humidification equipment as shown in FIGURE 8.

This process is valuable in equalizing the moisture content of wood. Theouter layer or sapwood of the tree contains a much larger percentage ofmoisture than does the inner or heartwood portion. When a wood productis manufactured, a single piece often consists of mixed heartwood andsapwood. This situation creates a drying problem for if conditions oftemperature and velocity are established to quickly dry the sapwood tothe desired mois-' ture content, the heartwood will often be over-dried.Conversely, if the heartwood is quickly dried to the desired moisturecontent, the sapwood will often remain wet.

The moisture content of Wood exposed to air ordinarily is dependent uponthe temperature and'relative humidity of the air. This moisture contentbalance of wood with the surrounding atmospheric conditions is known asits equilibrium moisture content. 'By selecting a specific temperatureand relative humidity, it is possible to bring all portions of a woodproduct ultimately to the desired equilibrium moisture contentregardless of the ini-tial moisture content of the wood. For example, ifan atmosphere of 186 F. dry bulb and 156 F. wet bulb is maintained, woodwill reach an equilibrium moisture content around minutes.

sists of a mixture of sap and heartwood with moistures ranging from 106%to 35%; the maximum allowable temperature for the wood during theprocess is 350 F. and the desired final moisture content is 5%.

In dryer l0, drying section 12 would be at 650 F. at a'velocity of 9,000f.p.m., the exposure time 1.0 minute. Upon leaving this section, theheartwood would be about 2% M.C. with a surface temperature of 350 F.,and the sapwood would be at 35% M.C. with a temperature of 210 F. Thesection 13 conditions would be 325 F. at a velocity of 9,000 f.p.m. andan exposure time of 1.5 Upon leaving section 13, the heartwood would beat 0.75% M.C. with a temperature of 325 F. and the sapwood would be at10% M.C. with a temperature of 220 F. Section 14 conditions would be at210 F, 50% relative humidity and a velocity of 7,000 ipm. Exposure timewould be 1.5 minutes. In this third zone the heartwood would increaseits moisture content to approach 5% while the sapwood would continue todecrease its moisture content to approach the desired'level of 5%. Thetotal elapsed time would be 4.0 minutes.

This is longer than the optimum time for drying sapwood alone. However,the method permits drying of wood containing areas of both high and lowmoistures in the same piece in as little as one-fifth the time requiredby other methods and without damage to the low moismoisture present tobe higher than this in some cases, but whatever the moisture level, itwill rarely have a spread in values greater than this example.

Schedule I Dryer schedule for %-inch Douglas Fir heartwood veneer,moisture content: max, 43%; avg. 35%; mm. 27%

Zone Percent Mois- Time in ture Content Zone Zone (ruins) TemperatureVelocity, In Out f.p.m.

8 5. 2 350 F 9, 000 0. 4D 3 0. 80 1 0 25 q o 5. .5 3 7, 000 0. 20 0.804. 0 160 wet bulb- 0. 25 3.6 J 4. 5 4. 7 4 80 F. (cooling 5, 000 0.20 4.0 4. 3 l 3. o a. 9

Total time in dryer, 1.58 mins. Speed of dryer proportional to dryerlength.

Schedule ll Dryer schedule for K-inch Douglas Fir intermixed sap andheartwood veneer moisture content: max-150%, sapwood mode-115%;heartwood ave.35%; him-27% Total time in dryer, 5.38 mins.

Speed of dryer proportional to dryer length.

From the above it will be seen that this relatively simple method orprocess may be used quickly and uniformly to reduce the moisturecontent, raise the moisture content or balance the moisture content ofwood, simply by varying the temperature, velocity, and/or the relativehumidity of the drying medium, and controlling the time of exposure ofthe wood to said medium.

What we claim as our invention is:

1. The method of changing the moisture content of wood of generallyplanar configuration which comprises drying the wood by impinging a gasat a gas temperature ranging from about 350 F. to about 950 F. against amajor planar surface of the wood to be dried substantially normalthereto at a velocity ranging from about 3000 f.p.m. to about 11000f.p.m. to strip 005 the moistureladen boundary layer of air at the woodsurface to cause rapid evaporation of moisture from the wood surface andsimultaneously cool said surface to protect it from harm ordiscoloration and for a time sufiicient to reduce the moisture contentof the Wood to a desired level without raising the board temperatureabove 350 F., said stripped-off moisture being replaced by moisture fromthe interior of the wood thereby rapidly adjusting the moisturethroughout the wood without harmful effects.

2. The method of changing the moisture content of wood of generallyplanar configuration which comprises drying the wood by directing aplurality of individual small jets spaced from each other in alldirections of a gas at a temperature ranging from about 350 F. to about950 F. against a major planar surface of the wood to be driedsubstantially normal thereto at a velocity ranging from about 3000f.p.rn. to about 11000 f.p.-m. to strip off the moisture-laden boundarylayer of air at the wood surface to cause rapid evaporation of moisturefrom the wood surface and simultaneously cool said surface to protect itfrom harm or discoloration and for a time suflicient to reduce themoisture content of the wood to a desired level without raising theboard temperature above 350 F., said stripped-off moisture beingreplaced by moisture from the interior of the Wood thereby rapidlyadjusting the moisture throughout the wood Without harmful effects.

3. The method of changing the moisture content of wood of generallyplanar configuration which comprises drying the wood by directing aplurality of individual jets of a gas at a temperature ranging fromabout 350 F. to about 950 F. against a major planar surface of the woodto be dried substantially normal thereto at a velocity ranging fromabout 3000 f.p.m. to about 11000 fprn. to strip off the moisture-ladenboundary layer of air at the wood surface to cause rapid evaporation ofmoisture from the wood surface and simultaneously cool said surface toprotect it from harm or discoloration and for a time sufiicient toreduce the moisture content of the wood to a desired level withoutraising the board temperature above 350 F. and quickly removing said gasfrom the wood, said stripped-off moisture being replaced by moisturefrom the interior of the wood thereby rapidly adjusting the moisturethroughout the wood without harmful eflects.

4. The method of changing the moisture content of wood of generallyplanar configuration which comprises raising the moisture content of thewood by impinging a gas at a dry bulb temperature of from to 212 F.against a major planar surface of the wood to be treated substantiallynormal thereto at a velocity ranging from about 1000 f.p.m. to at least11000 f.p.m. for a time sufiicient and with the relative, humidity ofthe gas adjusted to establish a desired equilibrium moisture content.

5. The method of changing the moisture content of wood of generallyplanar configuration which comprises raising the moisture content of thewood by directing a plurality of spaced individual small jets of a gasat a dry bulb temperature of from 140 to 212 F. against a major planarsurface of the wood to be treated substantially normal thereto at avelocity ranging from about 1000 f.p.m. to at least 11000 f.p.m. for atime sufficient and with the relative humidity of the gas adjusted toestablish a desired equilibrium moisture content.

6. The method of changing the moisture content of wood of generallyplanar configuration which comprises raising the moisture content of thewood by directing a plurality of spaced individual small jets of a gasat a dry bulb temperature of from 140 to 212 F. against a major planarsurface of the Wood to be treated substantially normal thereto at avelocity ranging from about 1000 f.p.m. to at least -1 1000 f.p.m. andwith the relative humidity of the gas at least 35% and for a timesufficient to adjust the moisture content of the wood to a desiredlevel.

7. The method of changing the moisture content of wood of generallyplanar configuration which comprises a first stage of drying the wood byimpinging a gas at a gas temperature ranging from about 350 F. to about950 F. against a major planar surface of the wood to be driedsubstantially normal thereto at a velocity ranging from about 3000f.p.m. to about 11000 f.p.m. to strip 05 the moisture-laden boundarylayer of air at the wood surface to cause rapid evaporation of moisturefrom the wood surface and simultaneously cool said surface to protect itfrom harm or discoloration and for a time sufficient to reduce themoisture content of the wood to a desired level without raising theboard temperature above 350 F., said stripped-off moisture beingreplaced by moisture from the interior of the wood thereby rapidlyadjusting the it! moisture throughout the Wood without harmful effects;and a second stage of raising the moisture content of said wood byimpinging a gas at a dry bulb temperature of from 140 to 212 F. againstthe planar surface of the wood to be treated substantially normalthereto at a velocity ranging from about 1000 f.p.m. to at least 11000f.p.rn. for a time sufficient and with the relative humidity of the gasadjusted to establish a desired equilibrium 7 moisture content.

8. The method of changing the moisture content of wood of generallyplanar configuration which comprises a first stage of drying the wood bydirecting a plurality of individual small jets spaced from each other inall directions of a gas at a temperature ranging from about 350 F. toabout 950 F. against a major planar surface of the wood to be driedsubstantially normal thereto at a velocity ranging from about 3000f.p.m. to about 11000 f.p.m. to strip off the moisture-laden boundarylayer of air at the wood surface to cause rapid evaporation of moisturefrom the wood surface and simultaneously cool said surface to protect itfrom harm or discoloration and for a time sufficient to reduce themoisture content of the Wood to a desired level without raising theboard temperature above 350 F., said stripped-off moisture beingreplaced by moisture from the interior of the wood thereby rapidlyadjusting the moisture throughout the wood without harmful effects, anda second stage of raising the moisture content of said wood by impinginga gas at a dry bulb temperature of from 140 to 212 F. against the planarsurface of the Wood to be treated substantially normal thereto at avelocity ranging from about 1000 f.p.m. to at least 11000 f.p.m. for atime sufficient and with the relative humidity of the gas adjusted toestablish a desired equilibrium moisture content.

References Cited by the Examiner UNITED STATES PATENTS 835,843 1l/O6Baetz 34-l3.8 2,590,850 4/52 Dungler 34l60 2,758,386 8/56 Cobb 343l2,791,039 5 /57 Rosen'baum 34l60 3,099,541 7/63 Hildebrand 34l8 WILLIAMF. ODEA, Acting Primary Examiner.

NORMAN YUDKOFF, Examiner.

4. THE METHOD OF CHANGING THE MOISTURE CONTENT OF WOOD OF GENERALLYPLANAR CONFIGURATION WHICH COMPRISES RAISING THE MOISTURE CONTENT OF THEWOOD BY IMPINGING A GAS AT A DRY BULB TEMPERATURE OF FROM 140 TO 212*F.AGAINST A MAJOR PLANAR SURFACE OF THE WOOD TO BE TREATED SUBSTANTIALLYNORMAL THERETO AT A VELOCITY RANGING FROM ABOUT 1000 F.P.M. TO AT LEAST11000 F.P.M. FOR A TIME SUFFICIENT AND WITH THE RELATIVE, HUMIDITY OFTHE GAS ADJUSTED TO ESTABLISH A DESIRED EQUILIBRIUM MOISTURE CONTENT.